Now imagine a hypothetical future scenario in which those resources are effectively unlimited; in the sense that each person is perfectly capable of perceiving the need/desires of their prospective partners, and satisfying them as best as possible, with capacity to spare; in which you don't need to compete for reproductive capacity or material resources are plentiful.

I think that would only be possible if the whole human race had the attentional resources to be a group marriage. I'm not sure it makes sense to say that everyone could be that good at modelling everyone one else.

My imagination only extends to raising Dunbar's number to 300, and I think that even that would produce large but hard to specify social changes.